Copper coated uranium article



United States Patent COPPER COATED URANIUM ARTICLE Allen G. Gray, RockyRiver, Ohio, assignor to the United States of America as represented bythe United States Atomic Energy Commisison No Drawing. ApplicationJanuary 6, 1945 Serial No. 571,673

6 Claims. (Cl. 29-494) This invention relates to the application ofprotective coatings on metallic uranium. The products produced inpracticing this invention are useful in a neutronic reactor of thetypedescribed in the copending application of Fermi et al., SerialNumber 568,904, filed December 19, 1944, which became U. S. PatentNumber 2,708,565 on May 17, 1955.

Metallic uranium is an easily oxidizable metal which is readily corrodedby atmospheric oxygen and by aqueous solutions. In order to protect themetal from the injurious effects of such media, it is desirable toprovide a coating of some material which is less easily acted upon.

Many common protective coating materials are unsuitable for coatinguranium; lead forms a pyrophoric alloy with uranium and for this reasonits use is objectionable; some metals, such as tin, permit diffusion ofthe uranium through coatings of these metals with consequent impairmentof their protective value; other metals form with the uranium, verybrittle alloys containing many cracks and crevices which reduce theireffectiveness.

It is an object of the present invention to provide protective coatingsfor uranium which are not subject to the defects enumerated above. Afurther object is to provide a barrier coating for uranium over which itis possible to apply various protective coatings that normally would beunsuitable because of their behavior toward the base metal. A furtherobject of the invention is to provide highly protective electrodepositedcoatings on metallic uranium.

In accordance with the present invention metallic uranium is providedwith a protective coating comprising copper. The copper may be applieddirectly to the metal or to an intermediate coating of some othersuitable metal possessing the property of adhering firmly to uranium andto copper. Nickel may be used to advantage for this purpose.

The metallic uranium preferably is prepared for the application of thecoating by an etchant treatment of'the metal surface with an aqueousetching solution containing chloride ions, for example a hydrochloricacid solution or a molten hydrate of ferric chloride. Particularlysatisfactory treatments of this type involve anodic pickling of themetal in aqueous trichloracetic acid solution, or aqueous phosphoricacid solution containing about /2%. of hydrochloric .acid, to remove a/2 to one mil layer of the surface metal. Prior to the etchant treatmentthe metal may be cleaned by sand-blasting, pumice-scrubbing, treatingwith organic solvents to remove grease or oily impurities, or treatingwith a chemical solvent for the oxide film on the metal. Aqueous nitricacid solutions containing from 30% to 55% HNO by weight are especiallyuseful for this purpose. An additional short treatment with this reagentshould be applied after the etchant treatment in order to remove thecoating of oxide or oxychloride from the etched metal surfaces.

The metallic copper coating may be applied by electroplating fromconventional copper electroplating baths, such as acid copper, cyanidecopper or pyrophosphate copper plating baths. The copper coatings of theinven- 2,854,737 Patented Oct. 7, 1958 tion may vary from exceedinglythin coatings of the order of 0.000015 inch up to 0.001 inch or more.The thickness of the coating should be selected appropriately to suitthe subsequent treatment of the metal. Thus the thinnest coatings arenormally employed in combination with relatively heavy coatings of othercoating metals, whereas the heavy coatings are employed alone or incombination with additional metal coatings of moderate thickness.

The copper protective coatings are especially advantageous in that theyinhibit diffusion of metallic uranium into other metals separated fromthe uranium by the copper. Thus the copper is particularly valuable as abarrier for preventing diffusion of uranium into protective coatings oflead, zinc, tin, cadmium, aluminum, and their alloys, and other metalsof relatively low resistance to uranium diffusion.

In my U. S. patent application Serial No. 572,093 entitled, NickelCoatings and Method of Applying, filed January 9, 1945, I have describeduranium products provided with nickel protective coatings. Although thecopper coatings of the present invention are superior to nickel coatingswith respect to the barrier effect which they have upon diffusion ofuranium, they can be substantially improved from the standpoint ofcorrosion resistance by combination with nickel coatings. Highlysatisfactory combinations are disclosed and claimed in my applicationaforesaid. Other useful combinations are described and claimed herein.

Nickel provides coatings which adhere more firmly to uranium than coppercoatings and consequently coatings of maximum adherence may be obtainedby applying a first coating of nickel and a second coating of copperover the nickel. In this case the nickel is employed for its adhesiveability rather than its corrosion resistance.

Corrosion resistance may be secured by means of copper alone or by meansof corrosion-resistant coatings applied over the copper. For example,copper constitutes a suitable base for application of lead coatings,which provide protection against a wide variety of corrosive agents. Thecopper, by barring diffusion of uranium into the lead coatings,eliminates the possibility of the forma tion of a pyrophoriclead-uranium alloy which might otherwise be produced. Theuranium-copper-lead bond is also more secure than the uranium-lead bond,and a uraniumnickel-copper-lead bond is even more satisfactory.

While the protective value of lead alone is considerable, electroplatedlead coatings have a somewhat porous structure permitting diffusion ofcorrosive agents into the plating and consequently provide a lesseffective protection than is obtained with an impervious coating. By theapplication of a coating of copper and tin or copper and nickel over thelead platings their porosity can be reduced and their corrosionresistance increased.

An especially important improvement can be secured by electroplating athin coating of tin upon an electroplated copper coating and thenheating the tin to a temperature above its melting point but below themelting point of copper to cause the tin to diffuse into the pores ofthe copper plating. Very thin tin coatings, commonly termed fiash"coatings, the thickness of which may be from 0.000015 inch to 0.00005inch, are suitable. Tin impregnated copper coatings prepared in thismanner have been found to possess corrosion resistance many timesgreater than simple copper-tin coatings. This technique is applicable tosimple copper platings upon uranium as well as complex coatings such asthe series nickelcopper-lead-copper. In the latter case the fusiontreatment should be conducted below the melting temperature of the leadplating.

The copper coatings of the present invention have been found to be ofvery substantial value for use in combination with chromium as aprotective coating for uranium. While chromium has been found to producefirmly adherent coatings on smooth metallic uranium surfaces, thethrowing power of chromium electroplating solutions is very low; hence,irregularities such as cracks, crevices, pinholes and the like inthemetallic uranium surfaces are coated relatively slowly and themetallic uranium in such surface irregularities maybe appreciably actedupon by the electroplating solution before the elec'-' troplatedchromium layer attains protective proportions. The copper not only addsto the value of the finishedcoatings because of its barrier action butalso protects fissures in the uranium metal surface from the chromiumelectroplating bath during the early period of electroplating withExample 1 A machined uranium rod about 8 inches long and 1% inch indiameter is prepared for electroplating by dipping in trichlorethene toremove grease, sand-blasting lightly,

dipping in aqueous 35% HNO by weight at 30 C. for 4 minutes, rinsingwith cold water, etching by anodic treatment in aqueous 50%trichloracetic acid solution at between 50* C. and 60 C. for minutes at50 amperes per square footcurrent density, rinsing with cold water,dipping for 4 minutes in cold aqueous 35 HNO solution to clean theetched metal surface, then rinsing in' cold water.

The cleaned etched metal rod is immediately electroplated in a copperpyrophosphate electroplating bath comprising 75 grams per liter ofcopper as pyrophosphate at a current density of 25 amperes per squarefoot at 60 C. with air agitation until a copper plating about 0.001 inchthick is formed (about 60 minutes).

The uranium, treated as described, has a firmly ad he'rent, continuouscoating of copper, which for many purposes provides an adequate degreeof protection against corrosion.

Since the copper electroplating is not as impervious as desirable forcertain purposes itis improved in this respect by electroplating it inan aqueous tin electroplating bath comprising 90 grams per liter ofsodium stannate at a current density of 25 amperes per square foot untila tin plating about 0.0001 inch thick has formed (about 10 minutes),then rinsing, drying, and heating at 300 C. for about 3 hours inSilicone #200 (a stable liquid polysilicone) bath to cause the tin todiffuse into the pores of the copper plating.

Example 2 A uranium rod copper-plated as in Example 1 is given a 0.001inch nickel plating in an aqueous nickel sulfate electroplating bathcomprising 240 grams per liter of nickel sulfate heptahydrate, 45 gramsper liter of nickel chloride hexahydrate, and 30, grams per liter of'boric acid, at a current density of '25 amperes per square foot forabout 50 minutes. The uranium is thus,provided a protective coating ofimproved corrosion resistance.

Example 3 An extruded uranium rod about 4 inches 'long and 1.1 inches indiameter is prepared for electroplating by .4 v dipping in aqueous 36%HCl solution for 20 seconds,

then in aqueous 50% HNO solution for 15 seconds and finally in aqueous36% HCl solution for 2 seconds. It is rinsed in cold water andelectroplated in a nickel sulfate electroplating bath of the samecomposition as described in Example 2 for 15 minutes at about 25 amperesper square foot and ambient temperature to provide a nickel platingabout 0.0003 inch thick.

The coated rod is rinsed with water and electr plated in a pyrophosphatetype copper electroplating bath of the composition recited in Example 1at a current density of 25 amperes per square foot at 60 C. until aflash coating of copper about 0.00003 inch thick has formed.

The copper-plated rod is again rinsed and then electroplated in a leadsulfamate electroplating bath comprising about 250 grams per liter oflead sulfamate at 25 C. at a current density of 25 amperes per squarefoot until a lead plating about 0.02 inch thick has formed (about 5hours and 40 minutes).

This coating adheres firmly to the uranium. The copper provides afirmbond between the lead and. the. nickel and acts as a highlyefiicient blocking layer or barrier to difiusion of uranium into theouter lead layer. The thick lead layer provides very good protectionagainst many corrosive agents.

Still greater protection is secured by electroplating a tin coatingabout 0.00005 inch thick over the lead to reduce its porosity. The tinis applied in the manner per square foot current density for 5 minutes,and toapply the tin over this copper layer. As in the coppertincoatingsystem of Example 1 the tin serves to fill the pores of the copper andthis efiect is increased and the corrosion resistance of the coatingimproved by a fusion treatment at about 250 C. for about 3 hours.

A somewhat similar but less effective coating is se cured substituting anickel plating for the tin plating.

Example 4 The process of Example 2 is repeated but in place of the leadelectroplating step a zinc electroplating step is substituted using anelectroplating bath containing 360 grams per liter of zinc sulfate atambient tempera! ture and a current density of 25 amperes per squarefoot until a zinc layer about 0.003 inch thick is applied (about 2hours). The copper serves the dual function of improving adherence ofadjacent metal layers and barring diffusion of uranium.

Example 5 A uranium rod is copper-plated as described in Ex ample 1 butfor only about 30 minutes. Over the 0.0005 inch copper plating (afterrinsing) a chromium plating is applied by electroplating thecopper-plated rod in .a chromic acid bath containing 40.0 grams perliter of chromic acid at 50 C. and 250 amperes per squarefoot currentdensity until a chromium layer 0.0005 inch thick has formed (about 30minutes). A bright, smooth,

even, firmly adherent composite electroplating is thus a probe construedas limitations upon the invention, the scope of which is defined in theappended claims, wherein:

I claim:

1. A metallic uranium article protectively coated with successive layersof copper and lead.

2. A metallic uranium article protectively coated with successive layersof nickel, copper, and lead.

3. A metallic uranium article protectively coated with successive layersof nickel, copper, lead, and tin.

4. A metallic uranium article protectively coated with successive layersof nickel, copper, lead, copper, and tin.

5. A metallic uranium article having an electroplated coating of copperbeneath an electroplated coating of chromium.

6. A metallic uranium article having a coating of electroplated copperimpregnated with fused tin.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Trans. Electrochem. Soc., vol. 66 (1934), page 41.

1. A METALLIC URANIUM ARTICLE PROTECTIVELY COATED WITH SUCESSIVE LAYERSOF COPPER AND LEAD.